The invention is directed to a microwave oven comprising an oven cavity, a microwave source and a wave guide device connected thereto for supplying microwave energy from said microwave source to said cavity via two or more feed openings positioned at a distance from each other. The invention is also directed to a method for excitation of the cavity of a microwave oven with microwave energy via feed openings arranged at a distance from each other in a side wall of the cavity, and furthermore to a wave guide device for carrying out the method.
A general problem of microwave ovens is that the microwave energy has a tendency to establish an uneven heating distribution in the cavity, meaning that so called "hot" and "cold" spots establish at different places in the cavity. In turn this gives rise to an inferior cooking result, which specifically may be observed for goods of low thermal conductivity. The generally accepted explanation of this phenomenon is that so called standing wave patterns establish in the cavity, with the consequence that the electric field energy become distributed around bulges and nodes of said patterns, thereby giving rise to said "hot" and "cold" spots.
Several proposals for a solution of this problem are previously known and one can find a more detailed description of the problem and the prior art solution thereof by reference to, e.g. the U.S. Pat. Nos. 4,336,434 and 4,458,126. One solution uses a so called field stirrer, comprising from point of principle a metal wing provided in the cavity or in the microwave feed system in order to obtain a continuous change of said standing wave patterns or the power balance between the same. An improved cooking result may also be obtained by the use of a so called rotating bottom plate, on which the food is positioned and allowed to rotate during cooking, thereby providing a levelling out of the heating energy in the food.
Related methods for obtaining an improved cooking result use two or more feed openings for the microwave energy, also combined with different types of field stirrers, moving microwave reflectors and rotating bottom plate. "Dual feed" or "multiple feed" arrangements of this type are known from, e.g. the U.S. Pat. Nos. 3,364,332, 3,439,143, 3,742,177, 3,993,886, 4,133,997, 4,427,867 and 4,140,888.
Other solutions use a rotating antenna for feeding of the microwaves, being then usually positioned at the centre of the roof or bottom of the cavity. Examples of a solution of this type may be found in the SE patent specifications nos. 8006994-1 and 8700399-2, of which the last mentioned discloses a feed system that may be regarded as a combination of rotating antenna and "dual feed".
A serious complication and a ground for a varying quality of the cooking result is the fact that the microwave field distribution in the cavity is influenced by the load, that is the weight, the shape and the quality of the food and the vessel which is used for the food, as well as the position of the load in the cavity. In the case of said "dual feed" system this, as one example, may have the consequence under certain given load conditions, that feeding of microwave energy to the cavity practically takes place through only one of the feed openings, and thereby that intended equalization of energy is not obtained. Under such circumstances the persistent load mismatch between the cavity and microwave source, being usually a magnetron, causes energy to be reflected back to the magnetron and among other things influences the operation point thereof and brings with it a decreased microwave efficiency. Problems will also appear with respect to uneven heating due to screening.
Proposals which aim to decrease the degree of feedback or reflection of microwave energy to the microwave source are known from, for example, the U.S. Pat. Nos. 3,437,777 and 3,745,292. The solutions are based on among other things the use of so called directional couplers directing the reflected energy instead to a microwave load. Even if the very microwave source is protected by this measure, drawbacks also will appear due to the fact that microwave energy is lost in said load. Furthermore, arrangements of this type will increase the complexity of the oven construction and thereby produce increased costs.
Even if the prior art as mentioned above and as known from the recited patent specifications to a varying degree will contribute to a desireable microwave field distribution int he cavity, a cooking result which is completely satisfactory will nevertheless not be obtained, which in many cases is due to the strong dependency between the microwave field distribution and the actual load/food. Furthermore, the prior art solutions are very often relatively complicated with consequent increased manufacturing costs.